1. Field of the Invention
The present invention relates generally to the field of manufacturing dry ice. More specifically, the present invention is a venting mechanism and a method of retrofitting this venting mechanism onto existing dry ice pelletizers to increase production rates.
2. Background Information
Solid state carbon dioxide (CO.sub.2), otherwise known as dry ice, is used in a vast array of applications. For example, dry ice is used in the processing and preservation of meats and other foods. Dry ice is an ideal method of preserving food because it sublimes directly from its solid phase to its gaseous phase, leaving no color, odor, taste, or residue and causes no lingering deleterious effects on food. In cooling and preserving food, dry ice pellets are placed directly onto the food to rapidly cool it below some specified temperature to prevent spoilage, both during processing and prior to refrigerated storage.
Traditionally, dry ice is produced and distributed in blocks. These blocks are large and cumbersome, making them difficult to use for many applications, including food related applications. Therefore, dry ice is now commonly sold as pellets for easy handling and manipulation. Specialized machines, called dry ice pelletizers, are used to produce dry ice in pellet form.
In a dry ice pelletizer such as that disclosed in U.S. Pat. No. 4,780,119 to Brooke and assigned to TOMCO Equipment Co., liquid CO.sub.2 is injected into a chamber, known as the extrusion cylinder, and flashed at atmospheric pressure. In this process, a part of the LCO.sub.2 changes phase to a solid ("snow") while the remaining portion changes phase to CO.sub.2 gas. The CO.sub.2 gas exits the extrusion cylinder through a plurality of gas vents. The proportionate amount of gaseous CO.sub.2 versus snow depends on the pressure and temperature of the LCO.sub.2 that is fed into the extrusion cylinder and the surrounding temperature of the extrusion cylinder--the lower the pressure and temperature, the greater the amount of snow produced in the flashing process.
When LCO.sub.2 is flashed under ideal conditions at atmospheric pressure, approximately 48% of the LCO.sub.2 is changed to snow while approximately 52% of the LCO.sub.2 is changed to gas. Because the percent of snow formation is directly proportional to the pressure inside the extrusion chamber where flashing occurs, it is important the pressure inside the extrusion chamber be kept as close to atmospheric pressure as possible. Therefore, any gas vents on the extrusion chamber must be free from obstruction by the snow particles that can become lodged in the vents.
Once the snow is formed in the compression cylinder, a piston is used to compact the snow into a block of dry ice. After forming the block of dry ice, the piston further pushes the block of dry ice against a die located at the end of the extrusion cylinder, opposite the piston. The block of dry ice is extruded through the die to form dry ice pellets. The pellets fall out of the cylinder and are collected and packaged for distribution to consumers.
As in any commercial endeavor, the faster the rate of production, the "better" the machine. Many physical features of the Brooke machine (U.S. Pat. No. 4,780,119) limit the rate of dry ice pellet production. For example, the Brooke machine has only one injection port for injecting the LCO.sub.2 into the compression cylinder. Because the rate of snow formation directly depends on the rate of LCO.sub.2 injection into the extrusion cylinder, the speed of the Brooke machine is limited by the single injection port. In addition, the Brooke machine contains a limited number of venting holes for venting the CO.sub.2 gas formed as a result of the flashing process, thereby increasing the amount of time needed for the flashing process, resulting in a decreased production rate.
U.S. Pat. No. 5,845,516 to Allen increases the production rate of the Brooke machine by adding an additional injection port to the compression cylinder, thereby increasing the rate by which the LCO.sub.2 is injected and snow is formed. While U.S. Pat. No. 5,845,516 increased the injection rate of the LCO.sub.2, it did not increase the rate by which the CO.sub.2 gas is vented out of the injection port. If the CO.sub.2 gas cannot be quickly vented, then the pressure inside the compression cylinder increases and lowers the amount of snow formed in the flashing process. Thus, although U.S. Pat. No. 5,845,516 increased the LCO.sub.2 injection rate, the limited CO.sub.2 venting rate continues to restrict the production rate of U.S. Pat. No. 5,845,516.
U.S. Pat. No. 5,548,960 to Anderson et al. attempts to increase the venting rate by using two cylinders--one for flashing the LCO.sub.2 into snow and the other for compacting the snow into dry ice pellets. The cylinder used to flash the LCO.sub.2 is completely porous, allowing a 360.degree. venting area of the CO.sub.2 gas. However, the Anderson patent has both practical and commercial limitations.
First, dry ice pelletizers are expensive machines. In order to use an Anderson machine to increase production, a dry ice producer must retire all his TOMCO machines (used by a majority of dry ice pellet producers) and purchase the new Anderson machine. This usually is not an economically practical solution for increasing the production rate.
Second, the Anderson patent cannot physically operate as fast as it claims to operate. The Anderson machine is made of two cylinders, one for flashing the LCO.sub.2 and the other for compacting and extruding the snow. In order to increase production rate, not only must snow be formed quickly in the first cylinder, snow must be compacted and ice pellets extruded at that increased rate in the second cylinder. In practice, LCO.sub.2 can be flashed into snow much faster than snow can be compacted and formed into pellets. Thus, the extrusion cylinder in the Anderson machine forms a bottleneck in the production process. As a result, the dry ice pellet production rate is not significantly increased by the Anderson machine.
Finally, in the Anderson machine, snow is likely to become lodged in the venting holes of the venting cylinder. The Anderson venting cylinder is made of a porous plastic material having tortuous and irregular shaped air passages. For all dry ice pelletizers, as a result of the rapid rate of CO.sub.2 gas formation in the flashing process, snow is likely to be blown into the venting holes or passages and block the passages, thereby reducing the venting rate, increasing the pressure inside the extrusion cylinder, and lowering the dry ice production rate. The tortuous and irregular air passages of the Anderson venting cylinder is particularly likely to cause blockage of the vent holes. Thus, while the Anderson machine may promise a faster production rate, the venting hole blockage problem actually reduces the snow, and therefore dry ice pellet, production rate.